This invention relates to an aluminum electrode foil for an electrolytic capacitor and an electrolytic capacitor using the foil.
An aluminum foil conventionally used as an electrode material for aluminum electrolytic capacitors are treated by electrochemical or chemical etching in general to enlarge its effective area and increase the capacitance per unit area. To improve etching performance, various treatments such as removal of a surface layer of the aluminum foil, formation of an oxide film, annealing and so forth are carried out in most cases before the etching treatment.
However, raising a surface expansion ratio of the aluminum foil as much as possible by etching means formation of a large number of deep etching pits. This invites problems such as a drop of mechanical strength of the aluminum foil and its breakage in a formation process, etc. after etching. The foil is thickened to cope with these problems, but if the foil is thickened, another problem that a size of a capacitor becomes larger arises. For these reasons, a method that can satisfy requirements for the increase of capacitance of an aluminum electrode foil for an electrolytic capacitor simultaneously with improvement of strength has been desired.
In the technical background as described above, the applicant of the present invention has previously proposed a method of producing an aluminum electrode foil for an electrolytic capacitor, comprising steps of reducing an aluminum foil after etching the foil (Japanese Patent Laid-Open No. Hei. 6-204094) as a method capable of improving the strength of the foil without increasing the thickness while ensuring the increase of the capacitance. By this method, reduction of the etched foil can accomplish both increase of the capacitance per unit volume and improvement of the strength of the foil.
However, subsequent studies by the inventors have revealed that even when etched foils are reduced at the same draft, increase of the capacitance per unit volume and improvement of the strength cannot be occasionally obtained depending on etching conditions of the etched foils. From further studies, it was found that aluminum density of a reduced etching layer functions as a dominating factor for effect in increasing capacitance per unit volume and in improving strength of the foil.
The present invention has been completed on the basis of the findings described above and objects are to improve strength of an aluminum electrode foil for an electrostatic capacitor and to increase the capacitance per unit volume.
Another object of the present invention is to provide an aluminum electrode foil for an electrolytic capacitor that can securely accomplish improvement of the strength and increase of the capacitance.
It is still another object of the present invention to provide an aluminum electrolytic capacitor that is small and yet has large capacitance.
The objects of the present invention described above can be accomplished by an aluminum electrode foil for an electrolytic capacitor having a sponge-like etching layer on at least one of the surfaces thereof, wherein the sponge-like etching layer is reduced in a direction of thickness, and aluminum density of the reduced sponge-like etching layer `d2` is set from 0.8 to 2.2 g/cm.sup.3.
The objects of the invention can also be accomplished by an electrolytic capacitor using the above aluminum foil as an electrode material.
Other object, features and advantages of the present invention will become more apparent from the following description taken with the accompanying drawings.